Interactive Calculator · 2026
Pipe Water Volume Calculator
Find how much water a length of pipe holds. Enter the internal diameter, the run length and a fill flow rate — get the litres held, the litres per metre and the time to fill, flush or disinfect the line.
Water held across common pipe sizes at 10 m
Your pipe run
The bore of the pipe, not the outside diameter.
The full run you want to fill, flush or disinfect.
The rate at which the tap or valve delivers water — a typical mixer sits around 6–15 L/min.
Water held across pipe sizes
The same 10 m run holds far more water as the bore grows — volume scales with the diameter squared.
This is also the water wasted while you wait for hot water to travel from the heater to the tap — every long, fat, un-insulated run dumps its cold contents down the drain first. Keep hot runs short and insulated to cut the wait and the waste.
For disinfection or chlorination, this is the line volume you must fill with dosed water and then flush out fully afterwards.
How this is calculated
- Volume per metre = π × (d ÷ 1000)² ÷ 4 × 1000 = π × (25 ÷ 1000)² ÷ 4 × 1000 = 0.49 L/m.
- Water held = volume per metre × length = 0.49 × 10 = 4.91 L.
- Time to fill / flush = water held ÷ (flow ÷ 60) = 4.91 ÷ (12 ÷ 60) = 24.54 s.
Indicative volumes for concept planning — real bore varies with pipe class and fittings. Confirm against actual internal diameters and NBC 2016 Part 9 before dosing or procurement.
Frequently asked questions
- How does the pipe water volume calculator work?
- It treats the pipe as a cylinder. It converts the internal diameter from millimetres to metres, works out the cross-section area as pi times diameter squared divided by four, and multiplies by length to get the volume held, shown in litres. It also divides the volume held by the flow rate to estimate the seconds needed to fill or flush the run.
- Should I enter the internal or the outside diameter?
- Always use the internal diameter, the actual bore that carries water, not the outside diameter printed on the pipe. For the same nominal size the bore changes with pipe class and material, so a heavier CPVC or GI pipe holds less than a thin-walled one. If unsure, check the manufacturer datasheet for the real inside diameter of your pipe class.
- How does this help with hot water waste, flushing or disinfection?
- The volume held is the cold water you run off while waiting for hot water to reach the tap, so shorter, insulated runs waste less. It is also the line volume you must fill with dosed water and then flush out fully when chlorinating or disinfecting. Treat the figures as indicative for planning and verify bore and dosing against NBC 2016 Part 9 or a professional.
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